High-frequency amplifier

ABSTRACT

A bias circuit is provided for attenuating harmonic distortions of a signal having a simple construction and which can be applied to a high-frequency amplifier used in a communication device, such as a mobile telephone. The circuit reduces a voltage drop therein and thus provides a high-frequency amplifier having reduced power consumption and an increased operating efficiency. The high-frequency amplifier includes an amplifier circuit, an output matching circuit, and the bias circuit. In the bias circuit, a parallel circuit including a first transmission line and a first capacitor has one end connected between the amplifier and the output matching circuit. The other end of the parallel circuit is connected to a power source and is grounded via a second capacitor. In the circuit, the bias circuit can be short-circuited in a desired frequency band while being an open circuit in a frequency band of a signal to be amplified, hence attenuating the harmonic distortions without using a low pass filter.

TECHNICAL FIELD

[0001] The present invention relates to a high-frequency amplifier foruse in a communications device such as a mobile telephone.

BACKGROUND ART

[0002] A conventional high-frequency amplifier shown in FIG. 14generally includes a transmission line 1403 having one end connectedbetween an amplifier circuit 1401 and an output matching circuit 1402and the other end connected to a power source 1405 and is grounded via acapacitor 1404.

[0003] The transmission line 1403 is designed to have ¼the wavelength ofa frequency band of a signal be amplified by the amplifier circuit 1401.The capacitance of the capacitor 1404 is set to a level great enough toshort-circuit in the frequency band.

[0004] A bias current received from the power source 1405 is a directcurrent and thus does not flow through the capacitor 1404 but flowsthrough the transmission line 1403 to drive the amplifier circuit 1401.The amplifier circuit 1401 amplifies a signal in the frequency band andmay simultaneously generate harmonic distortions in a band of n timesgreater than the frequency band (where n is an integer). The capacitor1404 is short-circuited in the frequency band and the length of thetransmission line 1403 is equal to ¼the wavelength. As a result, thetransmission line 1403 is inverted in phase at one end and stays open,thus allowing the amplified signal to be received not by a bias circuit1406 but by the output matching circuit 1402.

[0005] In the frequency band of 2n times greater than the band, thecapacitor 1404 is short-circuited. Also, the length of the transmissionline 1403 is equal to ½the wavelength. Therefore, the bias circuit 1406functions as a notch circuit. As a result, the harmonic distortions at2n times the frequency band can be attenuated and hardly received by theoutput matching circuit 1402.

[0006]FIG. 15 illustrates a frequency response of a conventionalhigh-frequency amplifier including an amplifier circuit 1401 having anoutput impedance of 3.2-j5.7Ω in an output frequency band of 900MHz.

[0007] Since the bias circuit of the conventional high-frequencyamplifier develops an insufficient short-circuit across the capacitor atthe frequencies of second and third harmonics, a low pass filter needsto be connected after the matching circuit for attenuating the harmonicdistortions. This increases the overall size of the circuit, and reducesthe operating efficiency of the conventional high-frequency amplifierdue to a loss of the added low pass filter; thus increasing currentconsumption for producing a desired level of power.

SUMMARY OF THE INVENTION

[0008] A high-frequency amplifier includes (a) an amplifier circuit, (b)an output matching circuit, and (c) a bias circuit. The bias circuitincludes a parallel circuit constructed by a first capacitor and a firsttransmission line having a first end connected between the amplifiercircuit and the output matching circuit and a second end connected to apower source, and a second capacitor having a first end connected to asecond end of the parallel circuit and a second end grounded.

[0009] While remaining open in a frequency band of a signal to beamplified by the amplifier, the bias circuit is short-circuited in adesired frequency band. This allows undesired harmonic distortions inthe signal to be attenuated without using a low pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a circuitry diagram of a high-frequency amplifieraccording to Embodiment 1 of the present invention.

[0011]FIG. 2 is a circuitry diagram of another high-frequency amplifieraccording to Embodiment 1.

[0012]FIG. 3 is a frequency response diagram of the high-frequencyamplifier according to Embodiment 1.

[0013]FIG. 4 is a circuitry diagram of a high-frequency amplifieraccording to Embodiment 2 of the invention.

[0014]FIG. 5 is a circuitry diagram of a high-frequency amplifieraccording to Embodiment 3 of the invention.

[0015]FIG. 6 is a frequency response diagram of the high-frequencyamplifier according to Embodiment 3.

[0016]FIG. 7 is a circuitry diagram of another high-frequency amplifieraccording to Embodiment 3.

[0017]FIG. 8 is a circuitry diagram of a high-frequency amplifieraccording to Embodiment 4 of the invention.

[0018]FIG. 9 is a circuitry diagram of another high-frequency amplifieraccording to Embodiment 4.

[0019]FIG. 10 is a circuitry diagram of a further high-frequencyamplifier according to Embodiment 4.

[0020]FIG. 11 is a circuitry diagram of a further high-frequencyamplifier according to Embodiment 4.

[0021]FIG. 12 is a perspective view of a high-frequency amplifieraccording to Embodiment 5 of the invention.

[0022]FIG. 13 is a perspective view of a high-frequency amplifieraccording to Embodiment 6 of the invention.

[0023]FIG. 14 is a circuitry diagram of a conventional high-frequencyamplifier.

[0024]FIG. 15 is a frequency response diagram of the conventionalhigh-frequency amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

[0025]FIG. 1 is a circuit diagram of a high-frequency amplifieraccording to Embodiment 1 of the present invention. A parallel circuitis constructed by a first transmission line 103 and a first capacitor104 and has one end connected between an amplifier circuit 101 and anoutput matching circuit 102. The other end of the parallel circuit isconnected to a power source 105 and is grounded via a second capacitor106.

[0026] An operation of the high-frequency amplifier will be described.In the following description, the amplifier amplifies a signal in afrequency band f, and doubled and tripled frequency bands of the band fare expressed by a frequency band 2f and a frequency band 3f,respectively (a frequency band 4f, a frequency band 5f, and so on).

[0027] The first transmission line 103 has a length equal to ½thewavelength of the frequency band 3f. A capacitance of the firstcapacitor 104 is determined so that the parallel circuit including thefirst transmission line 103 may have a high impedance or an open in thefrequency band f. A capacitance of the second capacitor 106 is set to alevel great enough to be a short-circuit in a frequency band higher thanthe band f.

[0028] A bias current received from the power source 105, being a directcurrent, flows not in the first capacitor 104 and the second capacitor106 but in the first transmission line 103 to drive the amplifiercircuit 101. The amplifier circuit 101 amplifies a signal in thefrequency band f and may simultaneously generate harmonic distortions.As the parallel circuit including the first transmission line 103 andthe first capacitor 104 is an open in the frequency band f, theamplified signal is received not by the bias circuit 107 but by theoutput matching circuit 102.

[0029] Since the second capacitor 106 is short-circuited in thefrequency band 3f, and since the length of the first transmission line103 is equal to ½the wavelength, the bias circuit 107 functions as anotch circuit. This attenuates the harmonic distortions in a signal inthe frequency band 3f, which thus does not flow in the output matchingcircuit 102.

[0030] The length of the first transmission line 103 of Embodiment 1 is½the wavelength in the frequency band 3f. The length may be ½thewavelength in a frequency m×f (where n is a prime number not smallerthan 3). In this case, the amplifier attenuates the harmonic distortionsof a signal at n times the frequency band.

[0031] As shown in FIG. 2, an output matching circuit 202 may include athird capacitor 208, a first inductor 209, and a fourth capacitor 210.The third capacitor 208 has one end connected to the amplifier circuit101, the first transmission line 103, and one end of the first inductor209. The other end of the first inductor 209 is connected to one end ofthe fourth capacitor 210. The third capacitor 208 and the fourthcapacitor 210 have the other ends grounded. A bias circuit 207 mayexclude the first capacitor 104. The output matching circuit 202 mayhave an output impedance of 50Ω in the frequency band f. Since thesecond capacitor 106 is short-circuited in a frequency band higher thanthe band f, the first transmission line 103, the first capacitor 104connected in parallel with the first transmission line 103, and thethird capacitor 208 are regarded to connect in parallel with each otherin the frequency band f. This allows the first capacitor 104 and thethird capacitor 208 to be combined, and thus allows the first capacitor104 shown in FIG. 1 to be eliminated.

[0032]FIG. 3 illustrates a frequency response of the amplifier where anoutput impedance the amplifier circuit 101 is 3.2-j5.7Ω at f=900MHz. Asapparent, matching is achieved at the frequency band f, and the signalthus passes. In the frequency band 3f, the signal does not pass.

[0033] While the transmission line and the capacitors of Embodiment 1may be implemented by any applicable manner, the present invention isnot limited to details of the implementation.

[0034] A mobile communication device equipped with the high-frequencyamplifier of Embodiment 1 can attenuate the harmonic distortions withits simple circuitry arrangement thus having a reduced overall size andan improved performance.

Embodiment 2

[0035]FIG. 4 is a circuit diagram of a high-frequency amplifieraccording to Embodiment 2 of the present invention. A parallel circuitconstructed by a second transmission line 403 and a fifth capacitor 404has one end connected between an amplifier circuit 401 and an outputmatching circuit 402. The other end of the parallel circuit is connectedto a power source 405 and one end of a second inductor 406. The otherend of the second inductor 406 is connected to one end of a sixthcapacitor 407. The other end of the sixth capacitor 407 is grounded.

[0036] An operation of the high-frequency amplifier will be described.

[0037] The second transmission line 403 has a length equal to ¼thewavelength in the frequency band f. A capacitance of the fifth capacitor404 is determined so that the parallel circuit including the secondtransmission line 403 may have a high impedance or an open in thefrequency band f. A capacitance of the sixth capacitor 407 is set to alevel great enough to be short-circuited in a frequency band higher thanthe band f, while the capacitance does not affect an impedance conditionof each component in the bias circuit 408.

[0038] A bias current from the power source 405, being a direct current,flows not in the fifth capacitor 404 and the sixth capacitor 407 but inthe second transmission line 403 to drive the amplifier circuit 401. Theamplifier circuit 401 amplifies a signal in the frequency band f and maysimultaneously generate harmonic distortions. Since the parallel circuitincluding the second transmission line 403 and the fifth capacitor 404is an open in the frequency band f, the amplified signal flows not inthe bias circuit 407 but in the output matching circuit 402.

[0039] Since being an open in the frequency band f, the parallel circuitconstructed by the second transmission line 403 and the fifth capacitor404 has a capacitive composite impedance in a frequency band higher thanthe band f. The second inductor 406 has an inductance so that thecomposite impedance series-resonates in the frequency band n×f (where nis an integer not smaller than 2). Since the sixth capacitor 407 isshort-circuited in the frequency band n×f, the bias circuit 408functions as a notch circuit. Hence, the harmonic distortions in anamplified signal at the frequency band n×f, since being attenuated, isnot transferred to the output matching circuit 402. Since the secondtransmission line 403 has a shorter length than the first transmissionline 103 of Embodiment 1, a voltage drop due to the bias current isreduced, thus improving an efficiency of the amplifier.

[0040] While the transmission line and capacitors in Embodiment 2 may beimplemented by any applicable manner, the present invention is notlimited to the details of the implementation.

[0041] A mobile communication device equipped with the high-frequencyamplifier of Embodiment 2 can attenuate harmonic distortions with itssimpler circuitry arrangement and decrease the voltage drop due to thebias current, thus having a reduced overall size and an improvedperformance.

Embodiment 3

[0042]FIG. 5 is a circuit diagram of a high-frequency amplifieraccording to Embodiment 3 of the present invention. A third transmissionline 503 has one end connected between an amplifier circuit 501 and anoutput matching circuit 502. The other end of the third transmissionline 503 is connected to one end of a third inductor 504, one end of aseventh capacitor 505, and one end of a fourth transmission line 506.The other end of the third inductor 504 is connected to one end of aneighth capacitor 507. The seventh capacitor 505 and the eighth capacitor507 have the respective other end grounded. The other end of the fourthtransmission line 506 is connected to a power source 508 and is groundedvia a ninth capacitor 509.

[0043] An operation of the high-frequency amplifier will be described.

[0044] The third transmission line 503 has a length equal to ½thewavelength in the frequency band 3f. A capacitance of the seventhcapacitor 505 is set to a level great enough to be short-circuited in afrequency band higher than the band f. An inductance of the thirdinductor 504 is determined so that the parallel circuit including theseventh capacitor 505 may have a high impedance or an open in thefrequency band 2f. A capacitance of the eighth capacitor 507 is set to alevel great enough to cut a bias current. A total length of the fourthtransmission line 506 and the third transmission line 503 is equal to½the wavelength in the frequency band 2f. A capacitance of the ninthcapacitor 509 is set to a level great enough to be short-circuited in afrequency band higher than the band f

[0045] A bias current from the power source 508, being a direct current,flows not in the ninth capacitor 509, the seventh capacitor 505, and theeighth capacitor 507 but in the fourth transmission line 506 and thethird transmission line 503 to drive the amplifier circuit 501. Theamplifier circuit 501 amplifies a signal in the frequency band f and maysimultaneously generate harmonic distortions. As the bias circuit 510 isan open in the frequency band f, the amplified signal flows not in thebias circuit 510 but in the output matching circuit 502.

[0046] Since the ninth capacitor 509 is short-circuited, the thirdinductor 504 and the seventh capacitor 505 is an open. A total length ofthe third transmission line 503 and the fourth transmission line 506 isequal to ½the wavelength in the frequency band 2f, and the bias circuit510 thus functions as a notch circuit. Hence, the harmonic distortionsin an amplified signal in the frequency band 2f can be attenuated andcan not be transferred to the output matching circuit 502 to be output.

[0047] Similarly, since the seventh capacitor 505 is short-circuited,and since the length of the third transmission line 503 is equal to ½thewavelength in the frequency band 3f, the bias circuit 510 functions as anotch circuit. Hence, the harmonic distortions in an amplified signal inthe frequency band 3f can be attenuated and do not flow in the outputmatching circuit 502.

[0048] As a result, the bias circuit 510 interrupts the amplified signalin the frequency band f and can attenuate the harmonic distortions inthe signal in the frequency bands 2f and 3f.

[0049]FIG. 6 illustrates a frequency response of the amplifier where anoutput impedance of the amplifier circuit 501 is 3.2-j5.7Ω at f=900MHz.As apparent, the matching is achieved, in the frequency band f, and thesignal passes while a signal in the frequency bands 2f and 3f do notpass.

[0050] A length of the third transmission line 503 in Embodiment 3 isequal to ½the wavelength in the frequency band 3f. The length may beequal to ½the wavelength in the frequency band m×f (where m is a primenumber not smaller than 3). The total length of the third transmissionline 503 and the fourth transmission line 506 is equal to ½thewavelength in the frequency band 2f. The length may be equal to ½thewavelength in the frequency band n×f (where n is an integer of notsmaller than 2). If m>n, the high-frequency amplifier attenuates theharmonic distortions of the signal at m times and n times greater thefrequency band f.

[0051] The length of the third transmission line 503 of Embodiment 3 isequal to ½the wavelength in the frequency band 3f. The thirdtransmission line 503 may be replaced by a bias circuit, a parallelcircuit constructed by the second transmission line 403 and the fifthcapacitor 404 shown in FIG. 4. The parallel circuit has a capacitivecomposite impedance in the frequency band 3f. If the composite impedanceseries-resonates with the third inductor 504 in the frequency band 3fand series-resonates with the seventh capacitor 505 in the frequencyband 2f, the bias circuit 510 functions as a notch circuit in the bands2f and 3f. Since the third transmission line 503 may be shortened, thevoltage drop due to the bias current can be reduced thus improving anefficiency of the high-frequency amplifier.

[0052] The other end of the fourth transmission line 506 of Embodiment 3is connected to the power source 508 and simultaneously is grounded viathe ninth capacitor 509. As shown in FIG. 7, the fourth transmissionline 506 may be replaced by a parallel circuit constructed by the fourthtransmission line 506 and a tenth capacitor 712, and a fourth inductor713 provided between the ninth capacitor 509 and the parallel circuit.In this case, the total length of the third transmission line 503 andthe fourth transmission line 506 is shorter than ¼the wavelength in thefrequency band f. Since being capacitive in the band 2f, the compositeimpedance of the parallel circuit series-resonates with the inductanceof the fourth inductor 713. A capacitance of the ninth capacitor 509 isset to a level great enough to cut a bias current. As a result, the biascircuit 714 functions as a notch circuit in the frequency bands 2f and3f. Simultaneously, the fourth transmission line 506 remains short andcan reduce a voltage drop due to the bias current thus improving theefficiency of the high-frequency amplifier.

[0053] The transmission line and the capacitors in Embodiment 3 may beimplemented by any applicable manner, the present invention is notlimited to details of the implementation.

[0054] A mobile communication device equipped with the high-frequencyamplifier of Embodiment 3 can attenuate the harmonic distortions withits simpler circuitry arrangement and decrease the voltage drop due tothe bias current thus having a reduced overall size and an improvedperformance.

Embodiment 4

[0055]FIG. 8 is a circuit diagram of a high-frequency amplifieraccording to Embodiment 4 of the present invention. A fifth transmissionline 803 and a sixth transmission line 804 have respective first endsconnected between an amplifier circuit 801 and an output matchingcircuit 802. The other end of the fifth transmission line 803 isconnected to a power source 805 and is grounded via an eleventhcapacitor 806. Similarly, the other end of the sixth transmission line804 is connected to the power source 805 and is grounded via a twelfthcapacitor 807.

[0056] An operation of the high-frequency amplifier will be described.

[0057] Each of the fifth transmission line 803 and the sixthtransmission line 804 has a length equal to ¼the wavelength in thefrequency band f Each of the eleventh capacitor 806 and the twelfthcapacitor 807 has a capacitance great enough to be short-circuited in afrequency band f or higher.

[0058] A bias current from the power source 805, being a direct currentand being transmitted to neither the eleventh capacitor 806 nor thetwelfth capacitor 807, is separated into two currents, one in the fifthtransmission line 803 and the other in the sixth transmission line 804,and is then combined to flow in the amplifier circuit 801 to drive thecircuit 801. The amplifier circuit 801 amplifies a signal in thefrequency band f and may simultaneously generate harmonic distortions.Since the eleventh capacitor 806 is short-circuited in the frequencyband f and is an open, the signal is inverted in phase in the frequencyband f at one end of the fifth transmission line 803, which is thus anopen. This makes the amplifier signal not to flow in a first biascircuit 808. Similarly, the twelfth capacitor 807 is short-circuited inthe frequency band f and is an open, the signal is inverted in phase atone end of the sixth transmission line 804, which is thus an open. Thismakes the amplifier signal not to flow in a second bias circuit 809.Accordingly, the signal amplified by the amplifier circuit 801 is outputfrom the output matching circuit 802.

[0059] Since the eleventh capacitor 806 is short-circuited in thefrequency band 2f, and since the fifth transmission line 803 is equal to½the wavelength, the first bias circuit 808 functions as a notchcircuit. Hence, the harmonic distortions in an amplified signal in thefrequency band 2f can be attenuated and not flow in the output matchingcircuit 802. Similarly, since the twelfth capacitor 807 isshort-circuited in the frequency band 2f, and since the sixthtransmission line 804 is equal to ½the wavelength, the second biascircuit 809 functions as a notch circuit. Hence, the harmonicdistortions in an amplified signal at the frequency band 2f can beattenuated and not flow in the output matching circuit 802.

[0060] The bias circuits functions as notch circuits even in a frequencyband being an even number of times greater than the band f and, thus,have frequency responses exhibiting attenuation poles.

[0061] According to Embodiment 4, the notching effect can be doubled atthe same frequency band while the voltage drop is reduced to half. As aresult, the high-frequency amplifier can be improved in operatingefficiency and enhanced in the attenuation of the harmonic distortionsmainly at the frequency band 2f

[0062] The two bias circuits in Embodiment 4 include respectivecapacitors short-circuited in a frequency band higher than the band f.The capacitors may be a single capacitor as shown in FIG. 9. In thiscase, the bias circuits can be installed in a reduced area.

[0063] The two bias circuits in Embodiment 4 are identical inconstruction. At least one of them may be accompanied with the biascircuit 107 shown in FIG. 1. Since the bias circuit 107 exhibitsnotching effect in the frequency band 3f, the circuits attenuate theharmonic distortions in the band 3f as well as in the band 2f (the bands4f, 6f,...).

[0064] The bias circuit and the output matching circuit in Embodiment 4may be implemented by the bias circuit 207 and the output matchingcircuit 202 shown in FIG. 2, respectively. In this case, the harmonicdistortions in the frequency bands 2f and 3f can be substantiallyattenuated while the number of components used for the circuits isreduced.

[0065] At least one of the respective transmission lines in the biascircuits in Embodiment 4 may be replaced by either the bias circuit 408shown in FIG. 4 or the bias circuit 510 shown in FIG. 5. Alternatively,the bias circuit 714 shown in FIG. 7 may be added to reduce the voltagedrop due to the bias current This attenuates the harmonic distortionseven more in the frequency band 2f (4f, 6f,...) and also in thefrequency band 3f.

[0066] The respective transmission lines of the bias circuits inEmbodiment 4 are connected between the amplifier circuit 801 and theoutput matching circuit 802. The output matching circuit 802 may bereplaced by the output matching circuit 202 shown in FIG. 2, and the twobias circuits may be connected to respective ends of the first inductor209 as shown in FIG. 10. In this case, the connection between theamplifier 801 and the output matching circuit 202 can be shortened thusreducing a loss of the signal in the frequency band f.

[0067] If the bias circuit connected to the node between the firstinductor 209 and the fourth capacitor 210 is identical to the biascircuit 107 shown in FIG. 1, the capacitance of the first capacitor 104may be included in the fourth capacitor 210. In this case, the biascircuits can be installed in a reduced area.

[0068] The present invention is not limited to the details of thearrangement of the transmission lines and the capacitors in Embodiment4.

[0069] A mobile communication device equipped with the high-frequencyamplifier of this embodiment can attenuate the harmonic distortions withits simpler circuitry arrangement thus having a reduced overall size andan improved performance.

[0070] If the output matching circuit is implemented by the outputmatching circuit 202 including the third capacitor 208, the firstinductor 209, and the fourth capacitor 210 shown in FIG. 2, the firstinductor 209 may be connected in parallel with a fourteenth capacitor1113 to form a parallel circuit as shown in FIG. 11. This shortens aflowing line of the output signal thus reducing a loss of the signal.The parallel circuit, upon having a resonant point matched with thefrequency band 2f or 3f of the harmonic distortions generated in theamplifier circuit 801, to eliminate the harmonic distortions. Thiseffect may be applicable to any of Embodiments 1, 2, and 3.

Embodiment 5

[0071]FIG. 12 is a perspective view of a high-frequency amplifieraccording to Embodiment 5 of the present invention. A power amplifierintegrated circuit (PA-IC) 1202 and a chip capacitor 1203 are mounted ona dielectric substrate 1201, and transmission lines 1204 and inductors1205 are formed by an electrode pattern. These forms an equivalentcircuit to the circuits of Embodiments 1, 2, 3, and 4 describedpreviously.

[0072] An operation of the circuit at high-frequencies is identical tothose of Embodiments 1, 2, 3, and 4. Embodiment 5 allows transmissionlines, inductors, and electrode connections between other devices to befabricated in one step, thus contributing to cost reduction of thehigh-frequency amplifier.

[0073] While the transmission lines are formed by an electrode patternon the dielectric substrate, they may be replaced by chip inductors. Inthis case, the circuit can be installed in a reduced area.

Embodiment 6

[0074]FIG. 13 is a perspective view of a high-frequency amplifieraccording to Embodiment 6 of the present invention. The high-frequencyamplifier includes capacitor electrodes 1302 and inductor electrodes1303 on dielectric layers 1301. A power amplifier integrated circuit(PA-IC) 1304 is mounted on the uppermost dielectric layer, andcomponents are connected electrically. An equivalent of the circuit isthat of any of Embodiments 1, 2, 3, and 4.

[0075] An operation of the circuit at high frequencies is identical tothat of any of Embodiments 1, 2, 3, and 4. This embodiment allowstransmission lines, inductors, and capacitors to be formed in alaminated body including the stacked dielectric layers. This reduces thenumber of the components and contributing to cost reduction of thehigh-frequency amplifier.

[0076] While the transmission lines, the inductors, and the capacitorsare formed between the layers, some portions of them may be implementedby a chip construction or an electrode pattern on the same layer as thathaving the PA-IC 1304 mounted thereon. This allows the circuitry to bedesigned more freely.

INDUSTRIAL APPLICABILITY

[0077] A high-frequency amplifier of the present invention including anamplifier circuit, an output matching circuit, and a bias circuit. Thebias circuit is short-circuited in a desired frequency band while beingan open in a frequency band of a signal to be amplified by the amplifiercircuit. This attenuates harmonic distortions of the signal.

[0078] Also, at least two bias circuits connected to a common powersource reduce voltage drop in the bias circuits themselves, andaccordingly provides the high-frequency amplifier having an improvedoperating efficiency while decreasing its power consumption.

1. A high-frequency amplifier comprising: an amplifier circuit; anoutput matching circuit; and a bias circuit comprising: a firsttransmission line having a first end connected between said amplifiercircuit and said output matching circuit, said first transmission linehaving a second end connected to a power source; a first capacitorconnected in parallel with said first transmission line; and a secondcapacitor connected between said second end of said first transmissionline and a ground.
 2. A high-frequency amplifier according to claim 1,wherein said bias circuit further comprises a second inductor connectedin series with said second capacitor.
 3. A high-frequency amplifiercomprising: an amplifier circuit; an output matching circuit comprisinga first inductor having a first end connected to said amplifier circuit;a first capacitor connected between a second end of said first inductorand a ground; and a second capacitor connected between said first end ofsaid first inductor and said ground; and a bias circuit comprising: afirst transmission line having a first end connected to said amplifiercircuit, said first transmission line having a second end connected to apower source; and a third capacitor connected between said second end ofsaid first transmission line and said ground.
 4. A high-frequencyamplifier according to claim 3, wherein said output matching circuitfurther comprises a fourth capacitor connected in parallel with saidfirst inductor.
 5. A high-frequency amplifier comprising: an amplifiercircuit; an output matching circuit; and a bias circuit comprising: afirst transmission line having a first end connected between saidamplifier circuit and said output matching circuit; a first capacitorconnected between said second end of said first transmission line and aground; a second transmission line having a first end connected to saidsecond end of said first transmission line, said second transmissionline having a second end connected to a power source; a first inductorconnected between said second end of said first transmission line andsaid ground; a third capacitor connected in series with said firstinductor; and a second capacitor connected between said second end ofsaid second transmission line and said ground.